The Just War Doctrine is an outline set out by Catholic theologians setting forth the conditions under which a Catholic person can support a war. If these conditions are not met then the war is unjust and all Catholics are morally bound to oppose it however they can, including refusing to fight.

According to theologians like St. Augustine and St. Thomas Aquinas:

  • War must be declared by a competent authority.
  • There must be a just cause for engaging in war; that is, a grave wrong to be corrected or right to be defended.

    Pope Pius XII added:

    • Owing to the increasing destructiveness of weaponry (especially nuclear weapons), war can not be waged morally except as an act of self-defense.
    • The use of arms must not produce evils and disorders graver than the evil to be eliminated.
    • War must be waged only as a last resort after all peaceful means have been exhausted.
    • A war can be fought legitimately only if its purpose is to achieve a just end.
    According to many Catholic pacifists, this rules out pretty much all war, especially in modern times.